The mechanism of action of ACTH is only partially understood. Adrenal steroid production is rapidly activated and deactivated (approximately 2 min); such a process is considered too rapid to involve regulation at the level of translation. Regulation would, however, be compatible with protein modification. It is generally accepted that in the adrenal cortex ACTH stimulates membrane- bound adenylate cyclase activity which leads to an increase in intracellular cAMP and the activation of cAMP-dependent protein kinase followed by steroid synthesis. The role of other protein kinase such as Ca2+-regulated kinases, however, is not well understood as yet. The rate-limiting step in steroidogenesis is the conversion of cholesterol to pregnenolone by a cytochrome P450 enzyme system. There is as yet no evidence that the P450 for cholesterol sidechain cleavage is subject to modulation by a phosphorylation-dephosphorylation mechanism. ACTH is known to activate systems which increase the availability of cholesterol for steroid production; and although the latter systems are known to be modulated by phosphorylation-dephosphorylation mechanisms, none is known to be regulatory. In the ACB laboratory, the guinea pig is used as an animal model to explore the responsivity of different zones of the adrenal cortex to ACTH. In this model, ACTH does not stimulate cholesterol side-chain cleavage activity and steroid production in the inner zone, in contrast to the outer zone, while adenylate cyclase activation and cAMP formation are similar for the two zones. The activity of HMG-Co A reductase (rate-limiting in cholesterol synthesis) is stimuated by ACTH in the inner zone as it is in the outer zone. Examination of protein kinase activity and protein phosphorylation reveals that cAMP- dependent, Ca2+/phospholipid-dependent, and Ca2+/calmodulindependent protein kinase activites are significantly higher in the outer zone than in the inner zone, and protein phosphorylation induced by the three kinase systems in the two adrenocortical zones reveals notable differences in phosphoprotein patterns.